1874.] The Past History of owr Moon. 317 
decomposing action would be rapid, though not aided—as 
in the case of our primeval earth—by an excessively high 
temperature. ‘‘ The formation of the chlorides of the 
various bases and the separation of silica would go on until 
the affinities of the acid were satisfied.” ‘* At a later period 
the gradual combination of oxygen with sulphurous acid 
would eliminate this from the atmosphere in the form of 
sulphuric acid.” ‘‘ Carbonic acid would still be a large con- 
stituent of the atmosphere, but thenceforward (that is, after 
the separation of the compounds of sulphur and chlorine 
from the air) there would follow the conversion of the com- 
plex aluminous silicates, under the influence of carbonic 
acid and moisture, into a hydrated silicate of alumina or clay, 
while the separated lime, magnesia, and alkalies would be 
changed into bicarbonates, and conveyed to the sea in a state 
of solution.” 
It seems to me that it is necessary to adopt some such 
theory as to the former existence of lunar oceans, in order 
to explain some of the appearances presented by the so- 
called lunar seas. As regards the present absence of water 
we may adopt the theory of Frankland, that the lunar 
oceans have withdrawn beneath the crust as room was pro- 
vided for them by the contraction of the nucleus. I think, 
indeed, that there are good grounds for looking with favour 
on the theory of Stanislas Meunier, according to which the 
oceans surrounding any planet—our own earth or Mars, for 
example—are gradually withdrawn from the surface to the 
interior. And in view of the enormous length of the time 
intervals required for such a process, we must consider that 
while the process was going on the lunar atmosphere would 
not only part completely with the compounds of sulphur, 
chlorine, and carbon, but would be even still further reduced 
by chemical processes acting with exceeding slowness, yet 
effectively in periods so enormous. But without insisting 
on this consideration, it is manifest that—with very 
reasonable assumptions as to the density of the lunar atmo- 
sphere in its original complex condition—what would remain 
after the removal of the chief portion by chemical processes, 
and after the withdrawal of another considerable portion 
along with the seas beneath the lunar crust, would be so in- 
considerable in quantity as to accord satisfactorily with the 
evidence which demonstrates the exceeding tenuity of any 
lunar atmosphere at present existing. 
These considerations introduce us to the second part of 
the moon’s history,—that corresponding to the period when 
the nucleus was contracting more rapidly than the crust. 
VOL. IV. (N.S.) 25 
